Prior art I/O-integrated displays include those which enable input by means of capacitive coupling. In capacitive coupling, a signal is fed to a tablet electrode provided on a tablet as an input receiving panel. As the tablet electrode is capacitively coupled to a stylus electrode provided in an input stylus as input means, the stylus electrode detects the signal. Thus, an input, as well as its coordinates, is sensed.
In capacitive coupling, however, the stylus electrode picks up noise in some circumstances, falsely recognizing it as an input. Here does a problem arise that an input is falsely sensed with no actual input.
To address the problem, Japanese published unexamined patent application 7-64704/1995 (Tokukaihei 7-64704, published on Mar. 10, 1995) discloses a technique attempting to avoid false sensing: an input stylus 101 has a stylus electrode 102 which is provided movable in the projection directions and a switch 103 which is turns on/off in accordance with the movement of the stylus electrode 102, as illustrated in FIGS. 17(a) and 17(b).
According to the technique, the switch 103 is off (an input is being made; FIG. 17(a)) when the stylus electrode 102 is pressed to the tablet 104 and on (no input is being made; FIG. 17(b)) when the stylus electrode 102 is distanced from the tablet 104. When the switch 103 is off, the stylus electrode 102 has a high input impedance due to a high-impedance resistor 105; when the switch 103 is on, the stylus electrode 102 has a low input impedance due to a low-impedance resistor 106 being inserted.
Thus, when the switch 103 is off, a high voltage level signal is detected through the stylus electrode 102; when the switch 103 is on, a low voltage level signal is detected through the stylus electrode 102. Accordingly, with such a configuration that an input is sensible when the detected signal has a high voltage level and not sensible when it has a low voltage level, no input is sensed when the stylus electrode 102 is picking up noise at a distance from the tablet 104. The technique can thereby avoid false sensing.
According to the technique, however, the switch 103 is off also when the stylus electrode 102 is pressed to an object other than the tablet 104. If the stylus electrode 102 picks up noise in such a state, a false sensing occurs as it did with older techniques.
The application further discloses more accurately sensing an input through discrimination between a detection signal and noise by comparing the detection signal with a predetermined threshold voltage. This indeed prevents false sensing if detected noise Ns is weaker than a threshold voltage Vth as shown in FIG. 18(a), but fails to do so if the detected noise level Ns is greater than the threshold voltage Vth as shown in FIG. 18(b).